As was suggested above, there are many possible explanations, none of them is dangerous or indicates a degradation of the material. I would simply wipe it off. I have several of these straps on my UN diver, one of them having been replaced many years ago. Sometimes, I have noticed a very thin film of humidity on the strap. It is the rubber strap with the smoothes surface in my strap collection, so I simply wiped the strap and used it.
Marcus